Sludge that is removed from water contains a number of constituents that can be divided approximately into inorganic and organic constituents. The inorganic constituents are primarily stones, gravel, sand, etc., while the organic constituents include wood and roots. The organic constituents also include oils that have been introduced into the water and have contaminated the latter. The sludge furthermore includes chemical contaminants, such as phosphates that have been introduced into the water and have become dissolved therein.
The objective of removing sludge from water is to rid the water of these inorganic and organic constituents. Actually, the greater problem arises as to what to do with these constituents after they have been removed from the water. Since dumps are becoming continuously more scarce, a further objective during the removal of sludge from water must be the recovery of inorganic and organic constituents, such as stone, gravel, said, wood, roots, etc., and the subsequent use of such matter as economically usable material. It should be necessary to place only a fraction of this matter in dumps.
Applicant's copending U.S. patent application, Ser. No. 060,746 Kreyenberg, filed June 10, 1987, discloses a method and apparatus for removing sludge from water. The objective of the disclosure of copending application Ser. No. 060,746 Kreyenberg is to improve the heretofore known methods and apparatus for removing sludge from water in such a way that small quantities only have to be placed in dumps, and that the rest of the constituents of the sludge can be provided for subsequent use as economically usable material. The method of the disclosure of Ser. No. 060,746 Kreyenberg, which includes separating the inorganic constituents in the sludge from the organic constituents thereof, comprises three process stages. In a first stage, those constituents that are larger than a given size are separated out from the sludge, with inorganic and organic ones of these constituents being separated from one another, whereupon these constituents are cleaned and then provided for subsequent use. In a second stage, inorganic constituents that are larger than a minimum size are separated out from the remaining matter, are cleaned, and are provided for subsequent use. Finally, in a third stage, water is removed from the inorganic and organic slurry of the remaining matter to form a thick sludge and effluent, which are withdrawn and/or further processed.
A drawback of the above-referenced process for removing sludge from water is that in the second stage the fine inorganic constituents, i.e. the mineral constituents, can be separated from the fine organic constituents only with great difficulty. In the cyclone unit used in this second stage, the density of the fine constituents is too low to make it possible to satisfactorily sort and seaprate out the inorganic constituents. The cyclone unit is intended to separate off the inorganic constituents that are larger than 0.063 mm and to supply them, for example, to a sand deposit, while the inorganic constituents that are smaller than 0.063 mm are supplied to the third stage. However, the rising water in the cyclone forces the lightweight inorganic constituents from the head of the nozzle, resulting in the drawback that sand particles are also drawn out and supplied to the third stage. This cannot be avoided without a precise separation of densities; but this is not possible with a cyclone. The relatively large sand particles are thus carried out in the third stage with the slurry water. In addition, some of the erosive sand passes into the water-removal mechanism (in the form of a centrifuge) of the third stage, where the sand causes undue wear. In addition, sand that could have an economical use in lost.
Another drawback is that if the cyclone unit is overloaded, no fractionation takes place any more. Instead, the material is washed out from the bottom and top along with the overflow water, without fractionation, and is supplied to a collecting tank that is disposed ahead of the water-removal unit of the third stage. The result is that the collecting tank is gradually silted up. Furthermore, some erosive sand again enters the water-removal unit of the third stage, where it causes wear.
Proceeding from the above, it is an object of the present invention to eliminate the drawbacks that occur during the second stage of the aforementioned method for removing sludge from water. It is a further object of the present invention to provide a facility for carrying out this method.